For Now, Pluto Holds Its Place in Solar System

In the hope of ending years of wrangling, a committee of astronomers and historians has proposed a new definition of the word “planet” that would expand at a stroke the family of planets from 9 to 12 and leave textbooks and charts in thousands of classrooms out of date.

But astronomers immediately began to wrangle about it.

“It’s a mess,” said Michael E. Brown of the California Institute of Technology.

Among the chosen few within the solar system would be not only Pluto, whose status has been challenged in recent years, but also Ceres, the largest asteroid; 2003 UB313, nicknamed Xena, an object discovered by Dr. Brown in 2005 orbiting far beyond Pluto in the outer solar system; and even Pluto’s largest moon, Charon.

In addition, at least a dozen more solar system objects are waiting in the wings for more data to see if they fit the new definition of planethood, which is that an object be massive enough that gravity has formed it into a sphere and that it circles a star and not some other planet.

The definition, they said, would apply both inside and outside the solar system.

The new definition was to be announced today in Prague, where some 2,500 astronomers are meeting in the triannual assembly of the International Astronomical Union. It is the work of the group’s Planet Definition Committee, whose chairman is Owen Gingerich, a Harvard astronomer. The astronomers will vote on the definition on Aug. 24.

In a statement, Dr. Gingerich said this might not be the last word on what a planet is. “Science is an active enterprise,” he said, “constantly bringing new surprises.”

So it was no surprise that as word of the decision leaked out yesterday, reaction from astronomers suggested that the argument was far from over.

“This will be the talk of the town in Prague,” said Alan P. Boss, a planetary theorist at the Carnegie Institution of Washington, who said the new definition, with four paragraphs and four footnotes, read as if it had been written by lawyers, not scientists. “I don’t think this is the one we're looking for.”

Neil deGrasse Tyson, director of the Hayden Planetarium, which was raked over the coals five years ago for demoting Pluto in an exhibit in its new Rose Center at the American Museum of Natural History in New York, was clearly disappointed in the committee’s work. “I’m happy there’s finally a definition that’s unambiguous,” Dr. Tyson said. “There hasn’t been one in 2,500 years.”

But roundness, he said, is not a very interesting attribute to use in classifying astronomical bodies. “A Plutophile is well served by this definition,” he said. “It is one of the few that allow you to utter Pluto and Jupiter in the same breath.”

But Alan Stern of the Southwest Research Institute in Boulder, Colo., called the definition “a nice solution that works both inside and outside the solar system.”

Everybody agrees that a little clarity is needed when it comes to categorizing the members of the solar system. The proposed definition would come as a relief to schoolchildren and others who have rallied to the cause of Pluto.

The planet (if that is what it is) has been an oddball ever since Clyde Tombaugh spied it wandering in the outer reaches of the solar system beyond Neptune in 1930. Not only is it much smaller than the other eight planets, only a fiftieth the mass of Earth, but its orbit is unusually elliptical and inclined to the plane that marks the orbits of the other planets. In recent decades, however, other objects with orbits like Pluto’s have been discovered in the Kuiper Belt, a junkyard of icy debris beyond Neptune.

Many astronomers began to argue that it made more sense to think of Pluto as a Kuiper Belt object, a minor planet instead of a planet. When it was reported that the Hayden Planetarium had done just that in its new Rose Center, which opened in 2000, a firestorm erupted. Schoolchildren rushed to the defense of lonely little Pluto.

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Two years ago, the International Astronomical Union appointed a group to come up with a definition that would resolve this tension. The group, led by Iwan Williams of Queen Mary University in London, deadlocked. This year a new group with broader roots took up the problem. After a sleepless night in Paris this spring, what Dr. Gingerich calls a miracle took place: “We had reached unanimous agreement.”

In a nod to the idea of classifying Pluto with the Kuiper Belt, the group proposed calling planets with elongated orbits beyond Neptune “Plutons,” while emphasizing that they would still be planets.

But Dr. Brown pointed out that at least 43 other publicly known objects in the Kuiper Belt were big enough to fit the planet definition, and that his group was sitting on a list of dozens more.

Dr. Boss said, “We’re going to have more planets inside the solar system than we have outside.”

He added, “Being a planet used to be an old boys’ club, with eight or nine members.”

Dr. Boss and Dr. Brown were especially critical of a feature of the new definition that would bestow planetary status on Charon, a moon of Pluto. With a diameter of about 700 miles, Charon is big enough for gravity to crush all other forces and make it round, but so are some of Jupiter’s and Saturn’s moons, as well as our own.

The difference, according to the definition, is that the center of gravity for Pluto and Charon is between them, not inside either one. So technically, Charon is not orbiting Pluto but is orbiting the center of gravity of the two bodies. The center of gravity for the Earth and its moon, on the other hand, is inside the Earth. Dr. Boss calls this “a legalistic definition.”

Dr. Stern, however, who is the principal scientist on the New Horizons space mission to Pluto, said the new definition was logical and not arbitrary.

It makes sense, he said, that there could be dozens of planets in the solar system. The new discoveries in the Kuiper Belt have put Pluto in context, he said. “Pluto is no longer the misfit,” Dr. Stern said. “It is closer to average than the Earth.”

He added: “Nature is much richer than our imagination. Life is tough, life is complicated. Get over it.”

Not everybody cares about the great planet debate.

Geoffrey W. Marcy of the University of California, Berkeley, a widely known hunter of planets around other stars, said in an e-mail message, “I am not attending the I.A.U. meeting, nor do I care about the outcome of any vote about whether Pluto and Xena are ‘planets.’ ”

“The universe,” Dr. Marcy added, “contains so much beauty and so many mysteries that we astronomers already have our hands full figuring out how it all came about.”

Correction: Aug. 18, 2006

A front-page article on Wednesday about a proposed redefinition of the word planet that would expand the solar system to 12 planets from 9 misspelled the given name of an astronomer at Queen Mary, University of London, who led a previous panel that debated the question. He is Iwan Williams, not Ivan.

Correction: Aug. 17, 2006

A front-page article yesterday about astronomers’ efforts to redefine the word planet misstated the date of a vote on a proposed definition that would increase the number of planets in the solar system to 12, from 9. The vote, by the International Astronomical Union, is scheduled for Aug. 24, not Aug. 25.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: for now, pluto holds its place in solar system. Order Reprints|Today's Paper|Subscribe